This invention relates in general to electrostatography, and, in particular, to a process for the production of improved ferrite electrostatographic carrier materials and their use.
The formation and development of images on the surface of photoconductor materials by electrostatic means is well known. The basic electrostatographic imaging process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as "toner." The toner will normally be attracted to those areas of the layer which retains a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing steps.
Several methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552, is known as "cascade" development. In this method, a developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically coated thereon is conveyed to and rolled or cascaded across the electrostatic latent image bearing surface. The composition of the carrier particles is so selected as to triboelectrically charge the toner particles to the desired polarity. As the mixture cascades or rolls across the image bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of our toner particles accidentially deposited in the background are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier and excess toner are then recycled. This technique is extremely good for the development of line copy images.
Another method of developing electrostatic latent images is the "magnetic brush" development process as disclosed for example, in U.S. Pat. No. 2,874,063. In this method, a developer material containing toner and magnetic carrier particles are carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carrier into a brushlike configuration. This "magnetic brush" is engaged with the electrostatic image-bearing surface and the toner particles are drawn from the brush to the latent image by electrostatic attraction. Thus, a developer mixture may be provided comprising a toner material and a carrier material which consists of particles which are magnetically attractable. Consequently, iron and magnetic ferrite materials have been employed as the carrier material in the electrostatographic arts.
Generally, in cascade or magnetic brush development typical carrier core materials include sodium chloride, ammonium chloride, aluminum potassium chloride, Rochelle salt, sodium nitrate, potassium chlorate, granular zircon, granular silicon, methyl methacrylate, glass, silicon dioxide, flintshot, iron, steel, ferrite, nickel, carborundum and mixtures thereof. Many of the foregoing and other typical carriers are described by L. E. Walkup in U.S. Pat. No. 2,618,551; L. E. Walkup et al. in U.S. Pat. No. 2,638,416 and E. N. Wise in U.S. Pat. No. 2,618,552. Generally, an average carrier particle diameter between about 30 microns to about 1,000 microns is preferred for electrostatographic use because the carrier particle then possesses sufficient density and inertia to avoid adherence to the electrostatic latent images during the cascade development process. In magnetic brush development, the ferrite carrier materials are generally homogenous, rounded or irregularly shaped particles having nominal particle sizes less than about 300 microns and more preferably between about 50 and 150 microns, the latter size range providing optimum image quality during extended use.
Ferrite materials are gaining ever increasing importance in the electronics industry and in the electrostatographic arts. Their use as low conductivity magnetic core materials and as carrier materials for photoconductive insulating materials is well known. Briefly, ferrites may be described in general as compounds of magnetic oxides containing iron as a major metallic component. Thus, compounds of ferric oxides, Fe.sub.2 O.sub.3, formed with basic metallic oxides having the general formula MFeO.sub.2 or MFe.sub.2 O.sub.4 where M represents a mono or divalent metal and the iron is in the oxidation state of +3 are ferrites. Ferrites are also referred to as ferrospinels since they have the same crystal structure of the mineral spinel MgAl.sub.2 O.sub.4. However, not all ferrites are magnetic such as, for example, ZnFe.sub.2 O.sub.4 and CdFe.sub.2 O.sub.4. This lack of magnetic property is due to the configuration of the ferrite lattice structure. Further, some ferrites, such as magnetobarite, BaFe.sub.12 O.sub.19, which exhibit permanent magnetic properties are referred to as "hard" ferrites. A "hard" ferrite is difficult to magnetize and demagnetize and thus is the type of ferrite that is desirable in a permanent magnet. A "soft" ferrite has the opposite property; it is easily magnetized and demagnetized. The "softer" the ferrite material is, the better it is suited to various electrical devices in which magnetization must be reversed very often per unit of time. If one plots the characteristics of a "hard" ferrite and a "soft" ferrite on a graph in which the imposed magnetic field forms the horizontal axis and the total magnetization forms the vertical axis, one obtains a characteristic curve resembling a thick S known as a hysteresis loop. A "hard" ferrite has a wide hysteresis loop and a "soft" ferrite has a thin one. Since each transversal of a loop represents energy lost, a narrow loop is desirable in devices in which magnetization must be reversed frequently.
The ferrite materials of main interest in the electrostatographic arts are the soft ferrites. The soft ferrites may further be characterized as being magnetic, polycrystalline, highly resistive ceramic materials exemplified by intimate mixtures of nickel, manganese, magnesium, zinc, iron or other suitable metal oxides with iron oxide. Upon firing or sintering, the oxide mixtures assume a particular lattice structure which governs the magnetic and electrical properties of the resulting ferrite.
In the past, ferrite materials have generally been prepared by dry and wet methods. The dry method involves the intimte mixing of pure oxides or carbonates of the desired metallic constituents and causing the mixture to react at elevated temperatures to form the desired structure. This method requires extensive ball-milling of the oxides or carbonates, usually dispersed in a liquid, until an efficient degree of mixing is obtained. The mixture is usually then dried, granulated, pre-sintered to form the desired structure, reground to attain a suitable particle size distribution, pressed or compacted with a binder material, and finally sintered or refired at temperatures above the pre-sintering temperature. The wet method generally involves the formation of an intimate mixture of the desired components by co-precipitation from solution. Usually, the components are dissolved as nitrates and co-precipitated as hydroxides, carbonates or oxalates. The product, after filtration and washing, is then prefired, reground, sized, compacted with a binder, and finally sintered or refired at temperatures above the pre-sintering temperature.
Several methods of preparing a manganese-zinc-ferrite are disclosed. For example, in U.S. Pat. No. 3,567,641, an oxide mixture is prepared, the mixture is pre-sintered at about 700.degree.-900.degree. C. for about an hour, the pre-sintered mixture is wet ground with CaO, the material is pressed to shape and sintered at 1,100.degree.-1,300.degree. C. for 1 to 4 hours in a low oxygen atmosphere, and then cooled in a substantially pure neutral atmosphere such as nitrogen. In U.S. Pat. No. 3,565,806 a ferrite material is produced by providing a mixture of the oxides, forming ferrite blanks from the oxide mixture, sintering the ferrite blanks at 1,200.degree.-1,300.degree. C. for about 4 to 20 hours, and during the last half of the sintering period the sintering occurs in an inert gas atmosphere containing less than 0.2 percent by volume of oxygen, and then cooling the sintered ferrite blanks to a temperature of about 300.degree. C. in the same inert atmosphere. In U.S. Pat. No. 3,839,029, Berg et al. teach a spray-drying process wherein a slurry of metal oxides is prepared in a liquid, the slurry is spray-dried to form spherical beads, and the beads are sintered to form ferrite beads. When employing a rotary kiln during the sintering step, a flow-promoting ingredient selected from aluminum oxide and zirconium oxide is mixed with the spray-dried beads to minimize bead-to-bead agglomeration and adherence of the beads to the furnace walls.
However, all of the aforementioned processes suffer from various disadvantages. More particularly, it has been found that thus-prepared ferrite materials when employed in an electrostatographic system for the development of electrostatic latent images are too sensitive to relative humidity changes to be acceptable for use in high speed electrostatographic devices employing magnetic brush development. One of the main reasons for poor performance of the ferrite materials at high humidity in the electrostatographic device was found to be the presence of certain species on the surface of the ferrite particles which changed surface conductivity and dielectric loss, and caused variations in charge relaxation of a developer mixture. The exact mechanism of this phenomenon and the identification of all contributing surface species is not now fully known. However, it has been found that surface sodium, perhaps combined with sulfate, is a major contributory contaminant. Surface zinc oxide has been found to be another major contaminant. Other contaminants may be calcium and potassium. However, sodium and zinc oxide are found to be present in by far the greatest amounts on the surface of ferrite materials prepared by prior art processes. It was found in machine testing that maximum acceptable levels of these surface species is about 20 parts per million for sodium and about 5,000 parts per million for zinc. The excess zinc oxide is usually due to the non-stoichiometry of the ferrite formulation. The major source of sodium contaminant was found to be due to its presence in the materials composition used and especially the deflocculant used in dispersing metal oxide slurries in the initial process steps.
Thus, previously known ferrite preparation processes and the resultant ferrite materials are deficient for the aforementioned reasons and undoubtedly due to lack of control of the surface properties of the finished product. More particularly, past ferrite material preparation processes and compositions were poorly controlled resulting in ferrites having humidity sensitive surface properties. Further, past ferrite preparation methods were deficient in the ability to control another surface property of ferrite known as BET surface area. BET surface area is measured as the area available for the adsorption of measurable quantities of gases and reflects the surface roughness of the powder product. For use as electrostatographic carrier materials, it is important to have the ability to control product BET area to whatever level is desired for maximum performance, and the ability to produce powder with uniform properties as to particle-to-particle. Since previously known ferrite preparation processes are deficient in one or more respects, there is a continuing need for an improved ferrite production process and for improved ferrite materials.